The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device suitably used for portable information terminals (for example, PDAs), mobile phones, car-mounted liquid crystal displays, digital cameras, PCs, amusement equipment, TVs and the like.
In recent years, liquid crystal display (LCD) devices have found widespread use for notebook PCs, mobile phones, information equipment such as electronic personal organizers, LC monitor-equipped camera built-in VTRs and the like, thank to their features of being thin and consuming low power.
A vertical alignment mode using a vertically aligned liquid crystal layer has increasingly received attention as a display mode permitting high contrast and a wide viewing angle. A vertically aligned liquid crystal layer is generally obtained using a vertical alignment film and a liquid crystal material having negative dielectric anisotropy.
For example, Japanese Laid-Open Patent Publication No. 6-301036 (Literature 1) discloses an LCD device in which a tilt electric field is generated around an opening formed in a counter electrode that faces a pixel electrode via a liquid crystal layer, so that liquid crystal molecules surrounding liquid crystal molecules existing in the opening, which are in the vertically aligned state, are aligned in tilt directions around the opening as the center, to thereby improve the visual angle characteristics.
However, in the device described in Literature 1, it is difficult to generate a tilt electric field over the entire region of each pixel. Therefore, each pixel has a region in which liquid crystal molecules delay in response to a voltage, and this causes a problem of occurrence of an afterimage phenomenon.
To solve the above problem, Japanese Laid-Open Patent Publication No. 2000-47217 (Literature 2) discloses an LCD device in which openings are provided regularly in a pixel electrode or a counter electrode, to form a plurality of liquid crystal domains each having radially tilt alignment in each pixel.
Japanese Laid-Open Patent Publication No. 2003-167253 (Literature 3) discloses a technique in which a plurality of projections are provided regularly in each pixel to stabilize the aligned state of liquid crystal domains having radially tilt alignment formed around the projections. This literature also discloses using a tilt electric field generated at openings formed in an electrode, together with the alignment regulating force of the projections, to regulate the alignment of liquid crystal molecules, and thus improve the display characteristics.
Japanese Laid-Open Patent Publication No. 2000-98393 (Literature 4) discloses a technology on an LCD device in which a groove structure is provided inside each pixel (at least inside a region to which a voltage for display is applied) to form axisymmetrically aligned domains in the regions of the pixel divided with the groove structure using the alignment regulating force of the side faces of the groove structure. When this technology is applied to a plasma-address LCD device, in which a voltage will be easily applied across this thick portion of the liquid crystal layer having the groove structure, lowering in drive voltage and improvement in response speed can be attained.
In recent years, a type of LCD device permitting high-quality display both outdoors and indoors has been proposed (see Japanese Patent Gazette No. 2955277 (Literature 5) and U.S. Pat. No. 6,195,140 (Literature 6), for example). In this type of LCD device, called a transflective (transflective) LCD device, each pixel has a reflection region in which display is done in the reflection mode and a transmission region in which display is done in the transmission mode.
The currently available transflective LCD devices adopt an ECB mode, a TN mode and the like. Literature 3 described above discloses adoption of the vertical alignment mode for a transflective LCD device, not only for a transmissive LCD device. Japanese Laid-Open Patent Publication No. 2002-350853 (Literature 7) discloses a technique in which in a transflective LCD device having a vertically aligned liquid crystal layer, the alignment (multi-axis alignment) of liquid crystal molecules is controlled with depressions formed on an insulating layer that is provided to double the thickness of the liquid crystal layer in a transmission region compared with that in a reflection region. This literature discloses that the depressions are in the shape of a regular octagon, for example, and projections or slits (electrode openings) are formed at positions opposing the depressions via the liquid crystal layer (see FIGS. 3 and 16 of Literature 7, for example).
To improve the display quality in the reflection mode, a technique of forming a diffuse reflection layer excellent in diffuse reflection characteristics has been examined. For example, Japanese Laid-Open Patent Publication No. 6-75238 (Literature 8) discloses a technique in which fine projections and depressions are formed randomly on the surface of a reflection electrode in a photolithography step using a two-layer photosensitive resin film to thereby obtain good diffuse reflection characteristics. Japanese Laid-Open Patent Publication No. 9-90426 (Literature 9) discloses a technique in which a reflection electrode having fine projections and depressions is formed by exposing a one-layer photosensitive resin film to light via a photomask for formation of contact holes and fine projections and depressions and developing the resin, for simplification of the fabrication process.
The technologies disclosed in Literature 2 and 3 have the following problems. In these technologies, projections or openings are provided in each pixel to form a plurality of liquid crystal domains in the pixel (that is, divide the pixel into domains), to thereby strengthen the alignment regulating force on liquid crystal molecules. According to examinations conducted by the inventors of the present invention, however, to obtain-sufficient alignment regulating force, the alignment regulating structure such as projections and openings must be placed on both sides of a liquid crystal layer (on the surfaces of a pair of opposing substrates facing the liquid crystal layer), and this complicates the fabrication process. The effective aperture ratio of each pixel having such an alignment regulating structure inside the pixel may decrease, and also the contrast ratio may decrease due to light leakage occurring in the peripheries of the projections in the pixel. The decrease of the effective aperture ratio and/or the decrease of the contrast ratio will become further eminent when the alignment regulating structure is provided on both substrates because the substrate alignment margin must be taken into consideration.
The technology disclosed in Literature 4, in which the groove structure is formed inside each pixel (at least inside a region to which a voltage for display is applied), has the problems that light leakage occurs near the inclined faces of the groove structure during non-voltage application, decreasing the contrast ratio, and that the effective aperture ratio decreases.
In the technology disclosed in Literature 7, it is necessary to provide projections or electrode openings at positions opposite to the depressions formed for control of the multi-axis alignment. This technology therefore has the same problems as those described above.
The fabrication process will be complicated if the methods disclosed in Literature 8 and 9 are employed to form reflection electrodes to improve the display quality in the reflection mode of a transflective LCD device, for example. That is, it is necessary to form fine projections and depressions for improving the diffuse reflection characteristics, in addition to the projections for alignment regulation, and this increases the cost of the LCD device.